1. Field of the Invention
The present invention relates to a tracking servo system and particularly to a method and apparatus for a tracking servo system wherein RF jitter is reduced by providing new circuitry rather than a low gear ratio.
2. Description of the Prior Art
An optical disc storage device can be either a device or system that is capable of retrieving information stored on an optical disc, or a device or system capable of recording information to and retrieving information from an optical disc. Examples of optical disc storage devices that are capable of retrieving information from an optical disc include compact disc (CD) players, video laser disc (LD) players and compact disc read-only-memory (CD-ROM) drives. Examples of optical disc storage devices that are capable of both recording information to an optical disc and retrieving information from an optical disc include recordable mini-disc (MD) players, magneto-optical (MO) disc drives and compact disc recordable (CD-R) drives.
Information is generally stored on an optical disc in the form of concentric or spiral tracks referred to as information tracks. In cases where information is already stored on an optical disc, the information tracks contain regions of optical contrast that represent the stored information. In the case of an unrecorded or blank optical disc containing pre-formatted tracks for recording information, a track that will become an information track may or may not have regions of optical contrast.
When an optical storage device is in its normal mode of operation, i.e. retrieving information from or recording information to an optical disc, the storage device rotates the disc while using a light beam emitted from a pick-up head to retrieve information from or record information to the disc. As the optical disc rotates, the pick-up head radially traverses the disc. This motion of the pick-up head and its direction are referred to as track following and track direction, respectively. When the pick-up head traverses the optical disc, a tracking servo system in the optical disc storage device keeps the beam of light emitted from the pick-up head centered on the information track, or the track that will become the information track in the case of recording information to a disc.
The optical disc tracking servo system is a closed loop in system that guides the pick-up head to follow the optical disc information track during normal operation. The tracking servo system readjusts the radial position of the pick-up head by sensing when the pick-up head or the light beam drifts off the center of the information track.
FIG. 1 is a cross-sectional diagram showing the pick-up head and spindle of an optical disc storage device. An optical disc 11 has a concentric or spiral information track 111 and is disposed on a spindle 12 which rotates the optical disc 11. A pick-up head 13 has a lens 131 and a fine actuator 132 which drives the fine track-following motion of the lens 131. A coarse actuator 14 drives a coarse track-following motion of the pick-up head when a light beam (not shown) emitted from the lens 131 cannot be centered on the information track 111 only with the actuator 132.
FIG. 2 is a block diagram showing a conventional tracking servo system in the above described optical disc storage device. The same elements in FIG. 1 and FIG. 2 are referred to by the same symbols. The tracking servo system 2 is a closed loop which comprises a fine controller 211, a fine driver 212, a fine actuator 132, a coarse controller 221, a coarse driver 222, a coarse 14 an optical sensor 23 and a pre-amplifier 24.
The optical sensor 23 senses displacement between the pick-up head and the center of the information track 111 and notes the displacement with a Tracking Error Signal TES. The signal TES is amplified by the pre-amplifier 24 and sent to the fine controller 211.
The fine controller 211 receives the amplified signal TES and accordingly generates a fine correction which is proportional to the displacement. The fine correction is sent with a fine correction signal FCS to the fine driver 212. The fine driver 212 receives the signal FCS and sends a fine driving signal FDS to the fine actuator 132 which accordingly generates a fine driving force to drive the track-following motion of the lens 131. The magnitude of the fine driving force is also linearly proportioned to the value of the fine correction sent with the signal FCS.
The coarse controller 221 receives the signal FCS sent by the fine controller 211. With a sampling rate lower than that of the fine controller 211, the coarse controller 221 low-pass-filters the signal FCS and generates a coarse correction which is sent with a coarse correction signal CCS. After receiving the signal CCS the coarse driver 222 sends a coarse driving signal CDS to the coarse actuator 14 which generates a coarse driving force to drive the track-following motion of the pick-up head. The magnitude of the coarse driving force is also linearly proportioned to the value of the coarse correction sent with the signal CCS.
Precisely speaking, the pick-up head is seldom centered on the information track by the above tracking servo system due to the friction of the gears. The track-following motion of the pick-up head is possible only when the coarse actuator generates a driving force stronger than the friction. This makes the essential displacement between the pick-up head and the center of the information track, i.e. the displacement cannot be zero, which results in RF jitter.
Theoretically, the above problem may be reduced to a small extent by increasing the gain of the coarse controller. This helps conquer the friction and makes a smaller displacement. However, to a liner closed-loop system, a large gain of the coarse controller tends to diverge the above tracking system and become unstable.
Traditionally, a low gear ratio is provided to solve the above problem but it elongates the access time of the optical disc storage device.